The present disclosure relates to metrology methods for measuring the focus variations during lithographic exposure of a photoresist, and reticles including a pattern for implementing the same.
A reticle, or a lithographic mask, comprises a transparent reticle substrate and a patterned optically opaque coating thereupon. Alternatively, a photomask may comprise a transparent reticle substrate with a partially transmissive layer, or with features etched out of the substrate to achieve optical phase shifting by changing the optical path length. The reticle is mounted into an exposure tool, which may be integrated into a tool called a scanner, so that radiation from a source of the exposure tool passes through the reticle and impinges on a photoresist applied to a top surface of a semiconductor substrate. The pattern of the reticle is thus transferred into the photoresist during the exposure so that the photoresist may have the same pattern as the pattern of the reticle after development. The reticle may be repeatedly employed to replicate the pattern in the reticle in the photoresist material on multiple semiconductor substrates. The coating on the reticle is optically opaque at the wavelength of the radiation source. Typical wavelengths of radiation that are employed for photolithography include 365 nm, 248 nm, 193 nm, 157 nm, etc. Such deep ultraviolet (DUV) wavelengths may be employed to pattern features having dimensions of 20 nm or greater in the photoresist.
In order to replicate the pattern of the reticles in a photoresist layer on a substrate with high fidelity, it is necessary that the distance between the lens of the exposure tool and the photoresist layer on the substrate be maintained at an optimal value, which is referred to as an optimal focus. A deviation of the distance between the lens of the exposure tool and the photoresist layer is referred to as a focus offset. Any non-zero value of the focus offset degrades the quality of the lithographic pattern formed in the photoresist layer by photographic exposure and development. The tolerance for the focus offset for forming a usable lithographic image is referred to as a depth of focus (DOF). The greater the DOF, the more immune a lithographic process employing a combination of an exposure tool and a reticle is to variations in the distance between the lens of the exposure tool and the photoresist layer, i.e., to the focus offset.
DOF has been continually decreasing with each generation of semiconductor technology. The DOF for advanced semiconductor technology is on the order of 100 nm as of 2012, and is expected to decrease in the future. In order to provide high yield lithographic processes, therefore, it is necessary to ensure that the distance between the lens of the exposure tool and the photoresist layer on the substrate be maintained at the optimal focus.